DELPHIN was a dolphin in the service of the god Poseidon. When his master was wooing Amphitrite and she fled, Delphin went in search of the nymph and persuaded her to agree to the marriage. For his service Poseidon placed him amongst the stars as the Constellation Delphinus.

Pseudo-Hyginus, Astronomica 2. 17 (trans. Grant) (Roman mythographer C2nd A.D.) :
"Dolphin. Eratosthenes [Greek writer C3rd B.C.] and others give the following reason for the dolphin’s being among the stars. Amphitrite, when Neptunus [Poseidon] desired to wed her and she preferred to keep her virginity, fled to Atlas. Neptunus sent many to seek her out, among them a certain Delphinus, who, in his wandering s among the islands, came at last to the maiden, persuaded her to marry Neptunus, and himself took charge of the wedding. In return for this service, Neptunus put the form of a dolphin among the constellations. More than this, we se that those who make statues of Neptunus place a dolphin either in his hand or beneath his foot--a thing they think will please the god especially.”

Oppian, Halieutica 1. 383 ff (trans. Mair) (Greek poet C3rd A.D.) :
“The Delphines (Dolphins) both rejoice in the echoing shores and dwell in the deep seas, and there is no sea without Delphines (Dophins); for Poseidon loves them exceedingly, inasmuch as when he was seeking Amphitrite the dark-eyed daughter of Nereus who fled from his embraces, Delphines (the Dolphins) marked her hiding in the halls of Okeanos and told Poseidon; and the god of the dark hair straightway carried off the maiden and overcame her against her will. Her he made his bride, queen of the sea, and for their tidings he commended his kindly attendants and bestowed on them exceeding honour for their portion.”